This invention relates to the art of packaging and cooling integrated circuits.
One commonly used integrated circuit package of the prior art is illustrated in cross section in FIG. 1 where it is identified by reference numeral 10. Some of the major components of this package 10 are a substrate 11, a lid 12, a plurality of conductive leads 13, and an integrated circuit chip 14.
As FIG. 1 shows, substrate 11 is rectangular in cross section. Also, it has a top major surface 11a and a bottom major surface 11b. Surface 11a has a cavity near its center which is shaped to receive chip 14. Surface 11b, by comparison, is merely flat.
A plurality of electrical conductors 11d lie within substrate 11 between the periphery of cavity 11c and the periphery of surface 11a. At the periphery of cavity 11c, electrical connections are made between the conductors 11d and chip 14 by a plurality of bonding wires 15. At the periphery of surface 11a, the conductors 11d contact the leads 13 directly which in turn extend from surface 11a to make electrical connections to an external system (not shown).
Lid 12 overlies cavity 11c and is rigidly attached to surface 11a at the periphery of the cavity by a lid attach material 16. This lid attach material 16 together with lid 12 and substrate 11 thus provide a hermetic enclosure for chip 14.
One specific example of the materials and their dimensions in the above-described circuit package is as follows: Substrate 11 is made of ceramic; and it has a length of 0.950 inches, a width of 0.950 inches, and a height of 0.060 inches. Chip 14 is made primarily of silicon; and it has a length of 0.300 inches, a width of 0.300 inches, and a thickness of 0.020 inches. Lid 12 is made of ceramic; and it has a length of 0.580 inches, a width of 0.580 inches, and a thickness of 0.030 inches. And the lid attach material 16 is made of a layer of glass having a thickness of 0.002 inches.
When chip 14 in package 10 is of the type that uses a relatively small amount of power (e.g., less than one watt), then no heat sink needs to be attached to the package. However, as the amount of power which chip 14 uses increases, a point is eventually reached at which a heat sink must be attached to the package in order to insure that chip 14 does not overheat.
Conventionally, the heat sink is made of metal, such as copper or aluminum; and it is rigidly attached by an epoxy or a solder to surface 11b directly below chip 14. During the attachment process, the epoxy or solder is heated to a fluid state whereupon it is dispersed in a thin smooth layer between surface 11b and the heat sink. Thereafter, the epoxy or solder is allowed to cool and harden.
However, this cooling and hardening step also induces stresses in the package, and particularly in the lid attach material 16. These stresses vary in magnitude with the overall shape of the particular heat sink that is being attached. And, depending on the shape of the heat sink, the stresses can become so large as to cause cracks in the lid attach material 16. When this occurs, the hermetic seal for chip 14 is broken which makes the package inoperable.
To further understand how the heat sink induces stresses in the lid attach material 16, reference should now be made to FIG. 2. That Figure contains a graph wherein the temperature of the integrated circuit package is plotted on a horizontal axis, and stress in the lid attach material 16 is plotted on a vertical axis. In this graph, a curve 21 illustrates how stress in the lid attach material 16 varies as a function of temperature under the condition where no heat sink is attached to package 10; while another curve 22 shows how stress in the lid attach material 16 varies under the conditions where a heat sink is attached to package 10.
Curves 21 and 22 begin at a temperature T.sub.LID, which is the temperature at which the lid attach material solidifies. For example, temperature T.sub.LID is approximately 320.degree. C. when the lid attach material is glass. In order to attach lid 12 to surface 11a, the lid attach material must be heated above temperature T.sub.LID, and typically it is heated to 420.degree. C. Thereafter, the package is cooled to room temperature T.sub.RT.
During this cooling, both substrate 11 and lid 12 contract. But this contraction induces only relatively small stresses in the attach material 16, becaues both lid 12 and substrate 11 are made of essentially the same material and thus they contact at nearly equal rates.
Thereafter, package 10 is reheated to attach the heat sink to surface 11b. In this step, the temperature to which the package is heated must exceed the solidification temperature T.sub.HS of the heat sink attach material. For example, the temperature for solder is about 183.degree. C., and for an epoxy is about 150.degree. C.
So long as the heat sink attach material remains liquid, the stresses induced in the lid attach material 16 remain relatively small. However, once the heat sink attach material solidifies at temperature T.sub.HS, the stresses in the lid attach material 16 rapidly increase. This rapid increase in stress is due to the fact that the heat sink contracts much more rapidly than the ceramic substrate. For example, the coefficients of thermal expansion for copper and aluminum respectively are about 2.6 and 3.6 times the expansion coefficient of ceramic.
As the heat sink contracts, it tends to compress that portion of surface 11b to which the heat sink is attached. This in turn causes substrate 11 to bend in an arc-shaped fashion as illustrated in FIG. 3. In this Figure, the amount of bending is greatly exaggerated merely to illustrate the point that such bending actually does occur. This bending, in turn, causes the rapid increase in stress in the lid attach material 16.
After the heat sink is attached and the integrated circuit package is placed in an operating environment, the package is subjected to some predetermined range of operating temperatures. A typical maximum operating temperature, for example, is 125.degree. C.; and a typical minimum operating temperature is -55.degree. C. Such maximum and minimum operating temperatures are indicated in FIG. 2 as T.sub.max and T.sub.min respectively.
Curve 22 shows that at temperature T.sub.max, the stress in lid attach material 16 is at a minimal level S.sub.min ; whereas at temperature T.sub.min, the stress in lid attach material 16 is at a maximum level S.sub.max. So in the operating environment, stress in the lid attach material varies between S.sub.max and S.sub.min. And the magnitude of the maximum stress S.sub.max as well as any cycling between S.sub.max and S.sub.min frequently cause lid attach material to crack.
In addition, when the heat sink attach material solidifies at temperature T.sub.HS, stress begins to occur at the interface between the heat sink and substrate 11. This stress, as with the stress in the lid attach material, is due to the fact that the heat sink contracts much more rapidly than substrate 11.
As the temperature decreases from T.sub.HS to T.sub.RT, the stress at the substrate-heat sink interface rapidly increases. Thereafter, when the heat sink and integrated circuit package are placed in an operating environment and cycled between the temperatures T.sub.min and T.sub.max, the stress at the substrate-heat sink interface cycles about a maximum value. Both the magnitude of that maximum value and the degree of cycling from it can produce cracks beteen the heat sink and substrate 11.